Leakage suppression tunnel for conveyorized microwave oven

ABSTRACT

A conveyorized microwave oven having two leakage suppression tunnels in series wherein the first tunnel includes a microwave choke and a second tunnel has a ferromagnetic layer covered with a smooth microwave transparent sheet. The choke tunnel presents a high impedance to microwave energy at the operating or fundamental frequency and the second tunnel attenuates spurious out-of-band radiation which propagates through the first tunnel and is typically concentrated at the harmonics. The ferromagnetic layer absorbs the broad-banded microwave energy and the smooth microwave transparent layer such as Lexan provides a surface that is easily cleanable and acceptable for use in cooking food. The sides of the second tunnel can be pivoted to an opened position by hinges to provide easy access to the inside for cleaning.

BACKGROUND OF THE INVENTION

Conveyorized microwave ovens have been used in industry for many yearsto cook or thaw foods and provide heat for processing objects such asrubber and foundry cores. These ovens generally operate at 915 MHz or2450 MHz because these frequencies are within narrow frequency bandsdesignated by government agencies for such purpose. The intensity ofmicrowave energy permited to leak from domestic and/or industrialmicrowave heating systems is restricted. In the United States, forexample, the Department of Health and Human Services requires that themicrowave energy leakage from a domestic oven not exceed one milliwattper square centimeter in the factory or five milliwatts per squarecentimeter in the home. Further, the Occupational Safety and HealthAdministration requires a microwave energy exposure of less than tenmilliwatts per square centimeter. The International Microwave PowerInstitute has adopted a standard for intensity of microwave energyradiation leakage which is "less than ten milliwatts per squarecentimeter". Furthermore, the Federal Communication Commission hasregulations regarding the amount of out-of-band radiation permissible bya microwave oven. Accordingly, systems employing the use of microwaveenergy for processing of materials or cooking and thawing food mustinclude apparatus to prevent the leakage of microwave energy from theenclosure.

Many industrial microwave heating applications require that there be acontinuous access aperture into the cavity so that materials may betransported through the cavity by a conveyor to achieve high throughput.The suppression of microwave energy from these apertures has presentedproblems which are much more complex than a batch-type microwave ovenwhich can be sealed by use of a door.

One prior art approach to the suppression of microwave energy from aconveyorized microwave system is to position a tunnel extending from theaperture and line the tunnel with a lossy material that absorbs themicrowave energy as it propagates therethrough. The food or productpasses through the tunnel on a conveyorized system. One lossy materialused is foamed glass but this material is fragile, dirty, and smellyand, therefore, is not compatible with food processing. Furthermore, theloss of foamed glass is relatively low so that an extremely long tunnelis needed in order to have effective leakage suppression from arelatively high power cavity.

Another lossy material used is a fluid that can be pumped aroundmicrowave transparent conduits in the tunnel so that the heat resultingfrom absorption can be removed to a heat exchanger thereby reducing thetemperature of the tunnel. Although this approach has an advantage overfoamed glass in limiting temperature requirements of the tunnel, theplastic or glass tubes are easily broken. Also, the pumps and heatexchangers such as radiators are relatively expensive. Furthermore, thisapproach, like the foamed glass, requires that the tunnel be relativelylong to provide adequate suppression and the cross-section through thetunnel must be relatively small.

Another prior art approach to the problem is to use a plurality of thinmetal flaps that hang in a lossy wall tunnel. Product passing throughthe tunnel on a conveyor pushes the flaps aside. When the tunnelcrosssection has mutually orthogonal dimensions that are substantiallygreater than a free space wavelength of the microwave energy and whenproduct pushing aside the flaps is not sufficiently lossy, the flaps donot provide an effective seal.

All of the approaches described above require the microwave energyentering the tunnel to be absorbed by some lossy material. Accordingly,each of these approaches detracts from the efficiency of the overallsystem because the available microwave energy must be split between theproduct and the tunnel. An improved approach, such as that described inU.S. Pat. No. 4,227,063, uses a plurality of conductive posts to providean effective choke of microwave energy. Although this approach providesenhanced system efficiency and effective sealing at the fundamentalmicrowave frequency, the bandwidth of the choke is somewhat limited suchthat out-of-band harmonics and other spurious radiation propagatethrough. In view of the Federal Communications Commission regulations,the harmonics and other spurious radiation must also be prevented fromleaking from the cavity.

SUMMARY OF THE INVENTION

The invention defines a conveyorized microwave oven comprising aconductive cavity having at least one continuous access opening, amagnetron for energizing the cavity with microwave energy, a firsttunnel having a first end connected to the outside of the cavity andsurrounding the access opening, the first tunnel providing a microwavechoke for the fundamental frequency of the magnetron, a second tunnelconnected to the second end of the first tunnel, the second tunnelcomprising means for attenuating a broad band of microwave energy, theattenuating means comprising a first layer comprising ferromagneticparticles bonded to at least a portion of the inner surface of thesecond tunnel and a second layer comprising a microwave transparentmaterial covering the first layer, and a conveyor system fortransporting product to be heated through the first and second tunnels.Typically, the oven would have access openings on opposing sides so thata conveyor belt could pass therethrough. In such circumstance, a pair oftunnels would be connected at each access opening. Preferably, the firstlayer comprises ferrite particulate dispersed in silicone and the secondlayer is a sheet of Lexan. The choke tunnel presents a high impedance tothe fundamental frequency of the magnetron which may typically be 915megahertz or 2450 megahertz. The ferromagnetic particles absorbbroad-band microwave energy in the second tunnel to suppress leakage ofharmonics and other spurious out-of-band radiation.

The invention also defines apparatus for suppressing microwave energyleakage from a conveyorized microwave oven cavity comprising a firstconductive tunnel connected to said cavity and surrounding the conveyorentrance to said cavity, said first tunnel comprising a choke having ahigh impedance at the fundamental frequency of the microwave energy, apair of substantially parallel metal slats extending horizontally fromthe first tunnel in a direction away from the cavity for forming thesides of a second tunnel, a first panel spanned between the bottom ofthe slats for forming the floor of the second tunnel, a second panelspanning between the tops of the slats and supported thereby for formingthe top of the second tunnel, the second panel having a first layerbonded to the underside of the second panel wherein the first layercomprises ferromagnetic particles for absorbing microwave energyescaping from the first tunnel into the second tunnel, the first layerbeing covered by a second layer of smooth microwave transparentmaterial, the second panel being pivotal about hinges for providingaccess to the second layer for cleaning, and a conveyor belt runningthrough the first and second tunnels for transporting objects into thecavity. Preferably, the first layer comprises ferrite particulatedispersed in silicone. Also, the second layer is preferably a sheet ofLexan. The edges of the slats may preferably be bent horizontallydefining a section parallel to the first and second panels.

The invention may also be practiced by the method of fabricating a wallfor a conveyorized microwave oven attenuating tunnel comprising thesteps of forming a shoulder around the edge of a horizontal metal panel,covering the upper surface of the panel with a priming agent, vacuummixing a viscous solution comprising ferromagnetic particles, a siliconeelastomer, and a catalyst, depositing the viscous solution onto thepriming agent on the panel, forming the viscous solution into a smoothsurface layer, and pressing a smooth microwave transparent sheet downover the layer thereby providing an easily cleanable covering for thelayer.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages of the invention will be understood morefully in the following detailed description thereof with reference tothe accompanying drawings wherein:

FIG. 1 is a perspective view of a conveyorized microwave oven utilizingleakage suppression tunnels;

FIG. 2 is a sectional view taken along line 2--2 of FIG. 1 showing thechoke tunnel;

FIG. 3 is a sectional view taken along line 3--3 of FIG. 1 showing theattenuation tunnel;

FIG. 4 is a side elevation view of the connection between the choketunnel and the attenuation tunnel of FIG. 1;

FIG. 5 is the first step in fabricating the attenuation tunnel;

FIG. 6 is the second step in fabricating the attenuation tunnel;

FIG. 7 is the third step in fabricating the attenuation tunnel;

FIG. 8 is the fourth step in fabricating the attenuation tunnel;

FIG. 9 is a sectional view of a choke tunnel that is an alternateembodiment of the tunnel shown in FIG. 2; and

FIG. 10 is a sectional view of an attenution tunnel that is an alternateembodiment of the tunnel shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown a conveyorized microwave oven 10having advantage for cooking or thawing food and providing heat forprocessing product such as rubber and foundry cores. The system includesa microwave cavity 11 formed by a plurality of bounded conductive walls13. Cavity 11 is energized with microwave energy by suitable means, suchas, for example, magnetron 12 which, consistent with governmentregulations, may preferably operate at 915 or 2450 megahertz. At leastone of the walls 13 of cavity 11 has an aperture 14 for providingcontinuous access to the processing region using conveyor belt 22. Morespecifically, it is preferable that opposing sides of the cavity haveelongated apertures 14 through which a continuous conveyor belt 22 ispassed. Conveyor belt 22 is driven by a motor 24 which is coupled to adrum 28 or cylinder which supports the conveyor. Chain 26 may be used tocouple the motor 24 to drum 28. The motor 24 is generally connected tothe output end of the conveyor belt 22 so as to pull the food or otherproduct through cavity 11. Accordingly, FIG. 1 is representative of theoutput side of cavity 11 where food or other product would be removedfrom the cavity 11.

Microwave leakage from cavity 11 is substantially eliminated by thecombination of choke tunnel 16 and attenuation tunnel 18. Choke tunnel16, which will be described later herein with reference to FIG. 2, has aflange 17 which is connected to wall 13 of cavity 11 around accessaperture 14 by suitable means such as spot welds or bolts which maypreferably be spaced less than 1.5 inches apart.

Choke tunnel 16 has two substantially parallel channels 20 or slatsextending outwardly in a direction away from microwave cavity 11.Channels 20 form the sides of attenuation tunnel 18. As will bedescribed and shown in detail later herein, attenuation tunnel 18further comprises a top and bottom which are connected to choke tunnel16 by hinges 19 and 46, respectively, so that attenuation tunnel 18 canbe opened for cleaning. It is understood that the input side to cavity11 with a choke tunnel 16 and an attenuation tunnel 18 would appearsimilar except that it would not have a motor.

Referring to FIG. 2, a sectioned view of choke tunnel 16 taken alongline 2--2 of FIG. 1 is shown. The tunnel is formed from four rigidconductive walls 29 which are connected to flange 17 by suitable meanssuch as welding. Flange 17, in turn, is mounted onto a wall of cavity 11around aperture 14 by suitable means such as spot welds 32 or bolts. Asis well known, conductive posts 30, suspended in rows and columns fromthe top wall of choke tunnel 16, provide an effective and substantiallyisotropic choke for microwave energy.

It is preferable that the conductive posts be approximately one-quarterwavelength long and the centers of closest adjacent posts beapproximately one-quarter wavelength apart. Accordingly, for a frequencyof 915 megahertz, posts 30 may preferably be approximately 3.22 incheslong and the centers of closest adjacent posts may preferably be spacedapproximately 3.22 inches apart. Posts 30 have diameters ofapproximately 0.375 inches although this dimension may not be critical.In one example of an operational configuration, tunnel 16 has a lengthof 26 inches and includes seven columns of posts, each spacedapproximately 3.22 inches apart as defined above. In that configuration,tunnel 16 has a width of 38 inches which includes 12 rows of postssimilarly spaced. It is preferable that the spacing between the bottomof the posts and the bottom wall of the tunnel be less than one-quarterwavelength. In the embodiment described above, the height of the tunnelis 6 inches so the spacing from the bottom of the posts to the bottomwall of the tunnel is approximately 25/8 inches. The choke tunnel 16described is very effective for sealing the escape of microwave energyat the fundamental frequency of approximately 915 megahertz. Thoseskilled in the art will recognize that the parameters and dimensions ofthe tunnel and posts can be modified. Also, the parameters anddimensions will be proportionally different for microwave ovensoperating at a frequency other than 915 MHz.

Referring to FIG. 3, there is shown a sectional view of attenuationtunnel 18 taken along line 3--3 of FIG. 1. Also referring to FIG. 4,there is shown a side elevation view of the connection between choketunnel 16 and attenuating tunnel 18. Attenuation tunnel 18 includes atop conductive panel 38 which spans across channels 20 or slats andforms the cover for attenuation tunnel 18. The edges 34 of top panel 38are bent downwardly and a layer 42 of plastic material having ferriteparticulate dispersed therein is bonded to the underside of panel 38.Layer 42 may preferably be covered with a sheet 44 of smooth, cleanablemicrowave transparent material such as Lexan. In the position as shownin FIG. 3, sheet 44 rests on the top horizontal sections 35 of channels20 and support layer 42 and panel 38. The end of top panel 38 adjacentchoke tunnel 16 includes a bracket 52 mounted perpendicularly theretowhich connects to flange 53 of choke tunnel 16 by hinges 19 whichincludes pins 48. Accordingly, for cleaning and inspection, attenuationtunnel 18 can be pivoted upwardly about pins 48 for easy access to theunder side of top panel 38 where layer 42 and sheet 44 are bonded. Asshown in FIG. 4, top panel 38 is partially pivoted.

Although a smooth, cleanable surface such as Lexan is necessary for foodprocessing, sheet 44 may be omitted in some industrial applications.However, a smooth inner surface of a high temperature plastic such asLexan is desirable because it can easily be cleaned. Other examples ofmicrowave transparent materials that could be used are polysulfone andacrylic resins.

Still referring to FIG. 3, a bottom conductive panel 40 forms the bottomwall of attenuation tunnel 18 and provides a surface for supportingconveyor belt 22. Although a layer 42 of microwave absorbing material isnot shown bonded to panel 40, that would be an alternate embodiment ifincreased attenuation were desirable. Like top panel 38, bottom panel 40has a perpendicular bracket 51 which is connected to flange 53 by hinges46 which include pins 50. As shown in FIG. 4, bottom panel 40 ispartially pivoted towards the opened position. Panel 40 which spansbetween the bottoms of channels 34 in the closed position are held inthat position by any suitable means such as a latch. The spacing betweensheet 44 and bottom panel 40 is preferably the same approximate spacingas between the bottom of posts 30 and the bottom wall of the choketunnel 16. Needless to say, the passage ways through tunnel 16 andtunnel 18 are vertically and horizontally aligned.

In operation, magnetron 12 is activated and energizes cavity 11 withmicrowave energy. As is conventionally done in industrial microwaveovens, the energy may be coupled to the cavity from a plurality ofdirective microwave radiators each coupled to an individual magnetronor, as an alternative, a single high power magnetron such as, forexample, 50 KW, may be used. Food or industrial product such as rubberor foundry cores are then fed in one end of the cavity on conveyor belt22 and removed from the other end. As is well known and as described inU.S. Pat. No. 4,227,063, choke tunnel 16 with conductive posts 30 asdefined herein substantially prevents the escape of microwave energy atthe fundamental frequency such as, for example, 915 MHz from cavityaperture 14. The choking structure as defined by the configuration ofconductive posts, however, is not very broad banded such that harmonicsor other spurious radiation is not effectively blocked by choke tunnel16. The ferrite particulate in layer 42 absorbs the microwave energythat is not effectively choked in choke tunnel 16. Accordingly, a safeenvironment is created for the operators of the oven and FederalCommunications Commission standards are met. Generally, otherferromagnetic particles such as iron could be used in place of theferrite particles but they may be more expensive and have a smallerbandwidth of microwave absorption. The power of the out-of-bandmicrowave energy entering attenuation tunnel 18 is relatively smallcompared to the power within cavity 11 because most of the energy is atthe fundamental frequency and sealed by choke tunnel 16; accordingly,there generally will not be a requirement to provide apparatus to removeheat from attenuation tunnel 18. Choke tunnel 16 doesn't heat because itisn't lossy.

The thickness of layer 42 may be varied so that the peak attenuation isnear the second harmonic where the chance of spurious radiation is thehighest. For example, for a 2450 MHz oven, the maximum attenuation wouldpreferably peak at about 4900 MHz which may result in a thickness of3/16 of an inch for layer 42. For a 915 MHz oven where the secondharmonic is 1830 MHz, a thickness of approximately 3/8 of an inch may bedesirable. For the preferred embodiment described heretofore and definedby a fabrication process discussed later herein, the attenuation hasbeen found to be on the order of 30 to 50 db per foot. This highattenuation is very desirable because the tunnels can be made relativelyshort thereby saving valuable factory floor space.

Referring to FIGS. 5-8, sequential steps in the fabrication of the topof attenuation tunnel 18 are illustrated. In the first step as shown inFIG. 5, the top panel 38 is formed with bracket 52 and the edges 34 ontwo sides. An example of an area is 34 inches long and 40 inches wide.Next, as shown in FIG. 6, the open ends are dammed up by suitable meanssuch as strips 58. It is preferable that strips 58 have a height ofapproximately the intended thickness of layer 42 so that they may beused as guides for attaining that thickness. More specifically, for thedescription of this particular embodiment for a 915 MHz oven, strips 58may preferably have a height of 3/8 of an inch above the surface area ofpanel 38. Also, an approximately 2-inch border 55 around the panel isrubbed with coarse emery cloth to obtain better adhesion of layer 42around the edges; it has been found that if the whole surface is sandblasted, the panel is more subject to warping. Before the step in FIG.7, the surface of panel 38 is primed with a thin coat of Dow Corning1200 primer or equivalent. Then, as shown in FIG. 7, the viscoussolution 60 forming layer 42 is poured onto panel 38. The solution isprepared by mixing approximately 12 pounds of Dow Corning Silastic Ewith approximately 1.2 pounds of its specified catalyst or curing agentin a vacuum mixer at an absolute pressure of approximately 2 inches ofmercury. It is stirred using an electric mixer while under vacuum tode-aerate the mix. This is an important step because air bubblesunderneath sheet 44 will weaken the bond between the two. After addingapproximately 31 pounds of Q1 ferrite as available from Indiana Generaland mixing thoroughly, the viscous solution 60 is poured onto panel 38in a fairly even layer leaving it slightly thicker on one side near anedge 34. The last of the solution should not be scraped from the mixingbucket because this gets too much air into the solution. A channelscreed is pulled across strips 58 from the side where the solution is alittle thicker thereby leveling layer 42. The thickness should beapproximately 3/8 of an inch and the strips 58 are used as guides.

Before the step of FIG. 8, a 1/16 inch thick Lexan sheet 44 is cut toapproximately 33.88 inches by 39.88 inches and one side is primed usinga cloth slightly dampened with Q3-6060 primer from Dow Corning. Thispriming should be done in advance because the primer should dry forapproximately one hour and the Lexan should be laid on layer 42 shortlyafter the layer is poured. The primed side of the Lexan should bedownward and should be pressed or rolled from one side to the otherforcing out air from underneath it. The Silastic E silicone cures inabout three days at 72° F. or one hour at 110° F; higher curingtemperatures may cause distortion. Then, panel 38 is inverted and hingedto flange 53 as described earlier herein.

Referring to FIG. 9, there is shown a sectional view of a choke tunnel70 which is an alternate embodiment of choke tunnel 16. Choke tunnel 70has advantage for use in curing rubber objects 72 in a 2450 MHzmicrowave oven. Choke tunnel 70 has an upside-down T-shape with conveyorbelt 74 running through the bottom wide section and oversized rubberobjects 72 passing through the central higher section. Choke tunnel 70is connected to the wall of the cavity by a plurality of spot welds orbolts on flange 76. The interior of the tunnel except for the sides ofthe bottom section is lined with conductive posts 78 which areapproximately one-quarter wavelength long and having centers of closestposts being spaced approximately one-quarter wavelength apart.Accordingly, at the frequency of 2450 MHz, conductive posts 78 areapproximately 1.2 inches long. The principle of operation of choketunnel 70 is similar to that of choke tunnel 16 and described withreference to FIG. 2.

Referring to FIG. 10, there is shown an attenuation tunnel 80 which isan alternate embodiment of attenuation tunnel 18 as described withreference to FIG. 3. The entire perimeter of the inside of tunnel 80 islined with a layer 84 of ferrite particulate dispersed in silicone suchas described with reference to layer 42 in FIG. 3. The layer of Lexan 44as shown in FIG. 3 is not present in the embodiment of FIG. 10 becausethis industrial application is for rubber processing and there is norequirement to have such a surface adjacent to the product. Furthermore,because there is no requirement to continuously clean the inner surfaceof attenuation tunnel 80, it has a flange 82 which is bolted to a flange(not shown) of choke tunnel 70. In other words, the two respectivetunnels are not hinged together so that attenuation tunnel 80 may bepivoted open.

When using a Lexan sheet 44 for food processing equipment such as shownin FIG. 1, the temperature of the Lexan must be maintained below 250° F.In an application for curing rubber such as shown in FIGS. 9 and 10 whenno Lexan is used, hot air may also be directed into the microwave cavityand the temperature of the air may therefore reach 350° to 400° F.

This concludes the description of the preferred embodiment. The readingof it will suggest many modifications and alterations without departingfrom the spirit and scope of the invention. Accordingly, it is intendedthat the scope of the invention be limited only by the appended claims.

What is claimed is:
 1. A conveyorized microwave oven, comprising:aconductive cavity having at least one continuous access opening; amagnetron for energizing said cavity with microwave energy; a firsttunnel having a first end connected to the outside of said cavity andsurrounding said access opening, said first tunnel providing a microwavechoke for the fundamental frequency of said magnetron; a second tunnelconnected to the second end of said first tunnel, said second tunnelcomprising means for attenuating a broad band of microwave energy, saidattenuating means comprising a first layer bonded to at least a portionof the inner surface of said second tunnel wherein the first layercomprises ferromagnetic particles; a second layer comprising a microwavetransparent material covering said first layer; and a conveyor systemfor transporting product to be heated through said first and secondtunnels.
 2. The oven recited in claim 1 wherein said first layercomprises ferrite particulate dispersed in silicone.
 3. The oven recitedin claim 2 wherein said second layer is a sheet of Lexan.
 4. Apparatusfor suppressing microwave energy leakage from a conveyorized microwaveoven cavity, comprising:a first conductive tunnel connected to saidcavity and surrounding the conveyor entrance to said cavity, said firsttunnel comprising a choke having a high impedance at the fundamentalfrequency of said microwave energy; a pair of substantially parallelmetal slats extending horizontally from said first tunnel in a directionaway from said cavity for forming the sides of a second tunnel; a firstpanel spanned between the bottom of said slats, said first panel formingthe floor of said second tunnel; a second panel spanning between thetops of said slats and supported thereby for forming the top of saidsecond tunnel, said second panel having a first layer bonded to theunderside of said second panel wherein said first layer comprisesferromagnetic particles for absorbing microwave energy escaping fromsaid first tunnel into said second tunnel, said first layer beingcovered by a second layer of smooth microwave transparent material; saidsecond panel being pivotal about hinges for providing access to saidsecond layer for cleaning; and a conveyor belt running through saidfirst and second tunnels for transporting objects into said cavity. 5.The apparatus recited in claim 4 wherein said first layer comprisesferrite particulate dispersed in silicone.
 6. The apparatus recited inclaim 5 wherein said second layer is a sheet of Lexan.
 7. The apparatusrecited in claim 4 wherein the edges of said slats are bent horizontallydefining sections parallel to said first and second panels.
 8. Themethod of fabricating a wall for a conveyorized microwave ovenattenuation tunnel, comprising the steps of:forming a shoulder aroundthe edge of a horizontal metal panel; covering the upper surface of saidpanel with a priming agent; vacuum mixing a viscous solution comprisingferromagnetic particles, a silicone elastomer, and a catalyst;depositing said viscous solution onto said priming agent on said panel;forming said viscous solution into a smooth surfaced layer; and pressinga smooth microwave transparent sheet down over said layer and bonding itthereto thereby providing an easily cleanable covering for said layer.